1. Field of the Invention
The invention disclosed herein relates generally to the field of heart defibrillator equipment. More specifically, the present invention relates to a special defibrillator apparatus which is suitable for use in implantable, automatic cardioversion systems, and which can generate particularly effective and beneficial high-voltage multiphasic defibrillation waveforms.
2. Description of the Prior Art
Ventricular fibrillation is almost always fatal unless promptly arrested. It has long been known that the application of a high energy pulse to the heart is often particularly effective in arresting this otherwise fatal condition and in restoring the synchronous operation of the heart muscles.
Automatic, implantable fibrillation sensors and defibrillation pulse generators are known in the art. See, for example, U.S. Pat. Nos. 3,614,954 and 3,614,955 to Mirowski et al., U.S. Pat. No. 4,254,775 to Langer, and U.S. Pat. No. 4,384,585 to Zipes. Such defibrillators, in order to be feasible, must occupy a minimal amount of space, be reliable in operation, and make efficient use of a depletable energy source.
It has been common for such prior art implantable defibrillators to generate unipolar type high-energy defibrillation pulses. See, for example, U.S. patent No. to Langer, U.S. Pat. No. Re. 30,387 to Denniston et al., U.S. Pat. No. Re. 30,372 to Mirowski et al., and U.S. Pat. No. 4,210,149 to Heilman et al. However, the use of unipolar pulses has been known to produce certain undesirable side effects including damage to the heart tissue near the electrode sites, induction of certain post-shock arrhythmias, and changes in the S-T segment. Moreover, under certain circumstances, such pulses are not effective to arrest ventricular fibrillation.
Recent medical research has shown that many of the problems associated with unipolar cardioverting pulses are alleviated or eliminated entirely when multiphasic cardioverting pulse trains are employed and that certain benefits are also obtained. For instance, it has been found that certain beneficial post-shock effects are imparted to the defibrillated heart by the trailing pulse of a three phase defibrillation waveform and that the effect vary with the level of energy imparted to the heart by this pulse. In addition, it has been found that the success rate in arresting ventricular fibrillation using three phase pulse waveforms is significantly greater than with unipolar pulses. See, for example, Schuder, Defibrillation of 100 kg Calves With Asymmetrical, Bidirectional, Rectangular Pulses, Cardiovascular Research 419-426 (1984), and Jones, Decreased Defibrillator-Induced Dysfunction With Biphasic Rectangular Waveforms, Am. J. Physiol. 247 (Heart Circ. Physiol. 16): H792-H796 (1984). Of course, many multiphasic waveform variations are possible and research is continuing in this area to discover others which may provide additional benefits and advantages in cardioverting and other applications.
A number of apparatuses for generating various forms of biphasic signals for pacing or defibrillation applications are known. One group of known apparatuses are manually-operated, electromechanical defibrillation pulse generators. These are not intended for and are totally unsuitable for use in automatic, implantable defibrillation systems, due to their size, mechanical nature, and high power requirements. See, for example, the biphasic defibrillation pulse generators described in U.S. Pat. Nos. 3,093,136 to Lohr, 3,241,555 to Caywod et al., and 3,359,984 to Daniher et al.
Another group of known apparatuses are biphasic pacing pulse generators such as those described in U.S. Pat. Nos. 3,924,641 to Weiss, 4,402,322 to Duggan, 3,563,247 to Bowers, and 3,946,745 to Hsiang-Lai et al. These known apparatuses have solved some of the problems of the electromechanical biphasic defibrillation pulse generators, but are not intended for and are not suitable for efficiently generating and applying to the heart the high-voltage pulses necessary to arrest ventricular fibrillation. In addition, the known pacing pulse generators lack the flexibility to generate the variety of multiphasic waveforms which medical research has recently shown to be advantageous in cardioversion applications, and to generate additional waveforms which continuing research may in the future discover to be beneficial. Moreover, these known generators provide no protection to the patient from internal malfunctions.
Accordingly, it is an object of the present invention to provide a highly energy efficient multiphasic pulse generator suitable for use in implantable automatic defibrillators.
It is another object to provide such a generator that is simple but flexible in its design and application and that can generate a variety of multiphasic waveforms.
It is still another object to provide a multiphasic defibrillation pulse generator that provides improved operational stability and accuracy independent of the magnitude of the pulses to be applied to the heart.
It is a further object to provide a multiphasic pulse generator that provides safeguards to the patient against internal malfunctions.